Controlled flight into terrain (CFIT) and collision with obstacles account for a large percentage of severe and fatal helicopter and other aircraft accidents, especially at night, adverse weather and otherwise bad visibility. The need for a device that would provide adequate warning against obstacles and CFIT for military aircraft is well known. The need also exists for commercial aircraft which are required to fly low, take off from, and land at unprepared and unknown sites, frequently at night and in adverse weather. These include medical evacuation (MEDEVAC), search and rescue (S&R) and police helicopters.
Many schemes have been devised in order to provide the required warnings. Most of these can be divided into two categories:
The first category includes systems which rely on an accurate navigation system, such as the global positioning system (GPS), and a stored data base of the terrain. An example is the enhanced ground proximity warning system (EGPWS) made by Honeywell. The main shortcoming of this category is that there is no assurance that the database is up to date and that there is no new obstacle (such as a pole) that has just recently been erected and does not appear in the database. Another shortcoming of the category is the reliance on GPS or a comparable navigation system, which may not always be available, or in the case of a military mission, might be jammed.
The second category includes systems which rely on real beam mapping, such as millimeter wave (MMW) or laser radars. Both MMW and laser sensors generally use mechanical scanning, which contributes to high cost, high weight and low reliability. Short wavelength (MMW or laser) requires expensive components. Furthermore, short wavelength radiation does not penetrate rain, fog, smoke and dust well and laser has a difficulty looking into, or close to the direction of the sun.
A method which uses radar with Doppler spectrum analysis, is described in Russian patent publication RU 2128846 This method provides the height of discrete obstacles, which is measured through the width of a Doppler spectrum. The Russian patent method does not describe 3D mapping of the terrain in front of the aircraft, nor does it address obstacles and terrain features which extend above the flight plane.
A method for elevation measurement, based on Doppler and azimuth is described in U.S. Pat. No. 5,847,673. The patent describes using a narrow, steerable antenna beam. It obtains object coordinates by first calculating it in antenna coordinates, then transforming it to aircraft or inertial coordinates. Mapping or terrain avoidance are not disclosed.
A method for SAR mapping around the line of flight is disclosed in European patent publication EP 0 434 064. This publication describes a scheme of suppressing a symmetrical range-Doppler cell, without the use of a null in the antenna beam, and without the need to average out the effect of residual signal from the “symmetrical” cell. It assumes that the cell is geometrically symmetrical to the cell of interest. This assumption is true when terrain is flat and is not inclined sideways relative to the flight plane (also when terrain is flat and horizontal). The assumption is not true for other terrain, meaning that the “symmetrical” cell may not be geometrically symmetrical and suppression of it may not be complete, resulting in an error in measuring the azimuth of the cell of interest. The method described does not measure elevation, and therefore cannot provide 3D mapping.
Definitions and Explanations of Certain Terms
The Line of Flight (LOF) is a line which coincides with the velocity vector of the aircraft. The aircraft's longitudinal axis (ALA), on the other hand, is a geometrical line that can be rigidly defined for the aircraft's fuselage. FIG. 1 shows the difference between the ALA 12 and LOF 14 for a helicopter 10.
LOF and ALA do not necessarily coincide. The angular displacement between the two is dependent on flying conditions. As an example, in the final stage of landing, the ALA may point above the horizon, while the LOF points below the horizon.
A horizontal plane view of ALA and LOF will show that they do not necessarily coincide in the horizontal plane either, again, depending on flight conditions. The most common cause for a sustained difference between the two in the horizontal plane is side wind, which causes the aircraft to drift aside.
While the ALA is rigidly tied to the airframe and its orientation can be supplied by horizontal and directional gyros (used for instrument flying) or otherwise by the navigation (NAV) system, LOF requires calculation. It can be supplied, for example, fully or partially from the aircraft's NAV system.
The flight plane (FP) is a plane determined by the LOF and a horizontal line perpendicular to it.
The term “FFT”, as used herein is used as a generic term to cover methods of spectral analysis of signals. It includes Fast Fourier Transform, other methods of spectral analysis or its equivalent coherent integration, for ease and simplicity of expression. It should be understood though that other ways to perform spectral analysis are also included and can be used in some embodiments of the invention.